Mr. Black said Mr. McCain and Ms. Iseman were friends and nothing more. But in 1999 she began showing up so frequently in his offices and at campaign events that staff members took notice. One recalled asking, “Why is she always around?”

That February, Mr. McCain and Ms. Iseman attended a small fund-raising dinner with several clients at the Miami-area home of a cruise-line executive and then flew back to Washington along with a campaign aide on the corporate jet of one of her clients, Paxson Communications. By then, according to two former McCain associates, some of the senator’s advisers had grown so concerned that the relationship had become romantic that they took steps to intervene.